Often, we end up both horrified and perplexed. What is with our species?

Well, research into the origins of violence offers clues but few definitive answers. For example, studies show that victims of child abuse are more likely to become abusers themselves, but about two-thirds do not. What makes the difference? Hoping to uncover the critical variables, researchers trace violent offenders' backgrounds. But, often enough, they find nothing that unusual.

What we do know is that violent behavior tends to fall into two primary groupings — reactive aggression and proactive aggression. The reactive type is driven by strong emotions and is often spur of the moment, such as in road rage and so-called crimes of passion. Anyone with a temper understands the impulse that drives reactive aggression, but most of us exercise sufficient self-control to stop short of violence.

The proactive variety is harder to comprehend. What is going on in the mind and soul of someone who carefully plans and executes a deliberate act of violence? In some instances, this type of aggression is found among so-called psychopaths who are bereft of a conscience and use their victims' suffering for personal gain, self-gratification or aggrandizement. However, there are also violent perpetrators who, in their own minds, rationalize their actions as justified in the service of some greater "good" (think terrorists).

In terms of psychopaths, neuroscientists have found that their brains often have certain suspicious abnormalities. However, some people with these same neurological deficits are not violent, so brain disorders don't tell the whole story. And should you wish to blame it on mental illness, those so afflicted are no more prone to violence than so-called normal folks.

Rather than looking inside the individual (neurology and psychology), social scientists look outside at conditions that beget violence. Factors such as poverty, lack of economic opportunity, discrimination, injustice and overcrowding, among others, are correlated with aggression. Still, most beset by these life circumstances do not act out in a violent manner.

So what is the genesis of this malady in our species? Is it a developmental history of abuse or bullying, a diseased brain, poor impulse control, ideological zealotry, or some invisible malevolence we refer to as evil?

We simply don't know. But whatever our explanations, the potential for violence is clearly a fundamental property of human nature, wherein we run up against a hard and sobering truth.

We humans are our own worst enemy.

Philip Chard is a psychotherapist, author and trainer. Names used in this column are changed to honor client confidentiality. Email Chard at pschard@earthlink.net or visit philipchard.com.